Presymptomatic Transmission of SARS-CoV-2 in Singapore

Presymptomatic Transmission of SARS-CoV-2 in Singapore. Spencer EA. Heneghan C.

Published on June 27, 2020

Reference Wei WE, Li Z, Chiew CJ, Yong SE et al. Presymptomatic Transmission of SARS-CoV-2 in Singapore.  MMWR Morb Mortal Wkly Rep 2020;69:411–415.
Study type
Country Singapore
Setting Community
Funding Details Non Reported
Transmission mode Presymptomatic transmission
Exposures Community, household contact

Bottom Line

Investigation of 243 cases of COVID-19 in Singapore identified seven clusters in which presymptomatic transmission was the most likely explanation for secondary infections.

Evidence Summary

The evidence of presymptomatic transmission in Singapore supports the likelihood that viral shedding can occur in the absence of symptoms and before symptom onset.

Seven COVID-19 epidemiologic clusters were identified for which presymptomatic transmission probably occurred. Within these clusters, ten such cases accounted for 6.4% of the 157 locally acquired cases. 

In the four clusters for which the date of exposure could be determined, presymptomatic transmission occurred 1 to 3 days before symptom onset in the presymptomatic source patient.

What did they do?

Clinical and epidemiologic data for all 243 reported COVID-19 cases in Singapore during January 23–March 16 were reviewed. Clinical histories were examined to identify symptoms before, during, and after the first positive SARS-CoV-2 test.

Study reliability

The study included all registered data on cases in Singapore in the relevant time period and is comprehensive in that way. These findings are also limited  as the possibility exists that an unknown source might have initiated the clusters.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Yes No Yes No Unclear

What else should I consider?

Despite strong surveillance and contact tracing systems, it remains a slight possibility that an unknown source might have initiated the clusters described. 

Recall bias could have affected the accuracy of symptom onset dates reported by cases, especially if symptoms were mild, resulting in uncertainty about the duration of the presymptomatic period. 

Underdetection of asymptomatic illness is expected. Recall bias and interviewer bias (i.e. the expectation that some symptoms would be present, no matter how mild), could have contributed to this.

Containment measures should account for the possibility of presymptomatic transmission by including the period before symptom onset when conducting contact tracing. These findings also suggest that to control the pandemic it might not be enough for symptomatic persons to limit their contact.

About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Elizabeth Spencer

Dr Elizabeth Spencer; MMedSci, PhD. Epidemiologist, Nuffield Department for Primary Care Health Sciences, University of Oxford.